1. Field of the Invention.
The present invention relates to a wheel alignment technique and in particular an improved technique for measuring caster and steering axis inclination.
2. Description of the Prior Art.
Proper alignment of the wheels of an automotive vehicle is important for proper handling of the vehicle and proper tire wear. In addition, proper alignment of the wheels will decrease fuel consumption as well as increase vehicle safety. The wheel alignment parameters which are measured and adjusted in order to achieve proper wheel alignment are camber, caster, steering axis inclination (SAI) and toe.
Camber angle is the inclination of the wheel plane with respect to vertical. It is considered positive when the wheel leans outward at the top, and is considered negative when it leans inward. Any wheel of an automobile can have camber.
Caster angle is the angle in side elevation between the steering axis of a steerable wheel (typically one of the front wheels) with respect to vertical. It is considered positive when the steering axis is inclined rearward (in the upward direction) and negative when the steering axis is inclined forward.
Steering axis inclination (SAI) or king pin inclination is the angle in the front elevation between the steering axis and vertical.
The static toe angle of a wheel, at a specified wheel load or relative position of the wheel center with respect to the sprung mass, is the angle between a longitudinal axis of the vehicle and the line of intersection of the wheel plane and the road surface. The wheel is "toed-in" if the forward portion of the wheel is turned toward a central longitudinal axis of the vehicle, and "toed-out" if turned away.
Camber, caster and SAI are typically measured using one or more inclinometer which are attached to the wheel. With camber, the inclinometer measures the angle that the plane of the wheel makes with the vertical. To measure caster, the wheel is turned through an arc, and the difference in the camber readings is used to derive the caster value. SAI is measured in a manner similar to caster, except that the inclinometer used for the SAI reading measures the change in inclination angle of a line in the plane of the wheel as the wheel is turned through an arc. The SAI measuring inclinometer is aligned at 90.degree. to the inclinometer used for reading camber and caster.
On an automotive vehicle, the two front wheels do not steer at the same rate in a turn. This is a requirement for proper handling of the vehicle during cornering. During cornering in an automobile, the wheel traveling along the inner arc of the turn must be at a sharper angle than the wheel traveling along the outer arc of the turn. Thus, when one wheel is turned to a precise toe-in angle, the opposing front wheel is not also turned to an equal toe-out angle. Previously, this has hampered measurements of caster and SAI angles because accurate measurements of camber and inclination angle for each front wheel had to be taken with the wheels at four precisely positioned toe angles. The steps in this process were positioning the left wheel precisely at a left toe-in angle and a left toe-out angle at which camber and inclination angles were measured and repeating the procedure by positioning the right wheel precisely at a right toe-out angle and a right toe-in angle at which measurements were taken of camber and inclination angles. Thus, camber and inclination angle measurements were taken at four precise toe angle values.
For example, the front wheels had to be steered left until the left wheel was positioned at the left toe-out angle equal to X. A first set of inclinometer readings from the left wheel would then be taken for camber (LC1) and inclination angle (LS1).
Next, the front wheels were turned further left until the right wheel was positioned at a right toe-in angle equal to X. Values were then recorded from inclinometer on the right wheel for camber (RC2) and inclination angle (RS2).
The front wheels were then turned to the right until the right wheel was positioned at the right toe-out angle equal to X. Values were then recorded from the inclinometer on the right wheel for camber (RC1) and inclination angle (RS1).
Finally, the wheels were turned further right until the left wheel was positioned at the left toe-in angle equal to X. In this position, additional readings were taken from the inclinometer on the left wheel for camber (LC2) and inclination angle (LS2).
Using common equations, caster and SAI can be calculated. For example: EQU CASTER=arctan((C1-C2)/(2 sin X)) Equation 1 EQU SAI=arctan((S1-S2)/(2 sin X)) Equation 2 (LT1
Using the above method of measuring caster and SAI, there are four steps each requiring positioning the wheels very precisely at each turn angle of the left (X) in and out toe angle, and the right (X) in and out toe angle. Furthermore, because the wheels must be precisely positioned four times, there is an increased probability that the measurements are taken at an erroneous wheel position.
The increased probability of errors entering the calculations in the above described method of measuring caster and SAI along with the problem that even a small error in an inclinometer reading will result in a significant error in caster and SAI calculations make the above method difficult, time consuming and expensive.
Thus, a new caster and SAI measurement technique in a wheel alignment system is desired that reduces the precision required in positioning the wheels and the number of times they must be positioned.